Method of fabrication of Al/Ge bonding in a wafer packaging environment and a product produced therefrom

ABSTRACT

A method of bonding of germanium to aluminum between two substrates to create a robust electrical and mechanical contact is disclosed. An aluminum-germanium bond has the following unique combination of attributes: (1) it can form a hermetic seal; (2) it can be used to create an electrically conductive path between two substrates; (3) it can be patterned so that this conduction path is localized; (4) the bond can be made with the aluminum that is available as standard foundry CMOS process. This has the significant advantage of allowing for wafer-level bonding or packaging without the addition of any additional process layers to the CMOS wafer.

CROSS-REFERENCE TO RELATED APPLICATIONS

Under 35 U.S.C. 120, this application is a Continuation Application andclaims priority to U.S. application Ser. No. 13/333,580, now U.S. Pat.No. 8,633,049, filed Dec. 21, 2011, which is a continuation of U.S. Pat.No. 8,084,332, issued Dec. 27, 2011, which is a continuation of U.S.Pat. No. 7,442,570, issued Oct. 28, 2008, which is related to U.S. Pat.No. 6,892,575, and related to U.S. Pat. No. 6,939,473, and related toU.S. Pat. No. 7,104,129 and related to U.S. Pat. No. 7,247,246, aportion of which is incorporated herein by reference.

FIELD OF THE INVENTION

The present invention relates generally to wafer bonding and moreparticularly to a method and system of bonding in a wafer packagingenvironment.

BACKGROUND OF THE INVENTION

MEMS technology has been under steady development for some time, and asa result various MEMS devices have been considered and demonstrated forseveral applications. MEMS technology is an attractive approach forproviding inertial sensors, such as accelerometers for measuring linearacceleration and gyroscopes for measuring angular velocity. A MEMSinertial sensor typically includes a proof mass which is flexiblyattached to the rest of the device. Relative motion between the proofmass and the rest of the device is driven by actuators and/or sensed bysensors in various ways, depending on the detailed device design. OtherMEMS applications include optical applications such as movable mirrors,and RF applications such as RF switches and resonators.

Since MEMS fabrication technology is typically based on processingplanar silicon wafers, it is useful to classify MEMS devices accordingto whether the actuation and/or sensing performed in an inertial sensor(or other application) is in-plane or out of plane (Le., vertical). Morespecifically, a device is “in-plane” if all of its sensing and/oractuation is in-plane, otherwise it is “vertical”. Thus MEMS devices areundergoing steady development, despite fabrication difficulties thattend to increase.

One approach which has been used to fabricate vertical MEMS devices ishybrid integration, where elements of a MEMS assembly are individuallyassembled to form the desired vertical structure. For example,attachment of a spacer to a substrate, followed by attachment of adeformable diaphragm to the spacer, provides a vertical MEMS structurehaving a spacing between diaphragm and substrate controlled by thespacer. U.S. Pat. No. 6,426,687 provides further information on thisapproach. Although hybrid integration can provide vertical MEMS devices,the cost tends to be high, since manual processing steps are usuallyrequired, and because hybrid integration is typically performed onsingle devices. Therefore, there is a need for reduced cost integratedMEMS devices that is unmet in the prior art.

CMOS compatible wafer-wafer bonding is very desirable forwafer-level-packaging. Its use has been demonstrated in a variety ofdifferent technologies. However, most of these processes have beenlimited to providing protection of a sensitive feature from post processhandling, such as sawing, die bonding, testing, package, etc.

The need for a robust wafer level integration that can allow forsimultaneous wafer-level-packaging and electrical interconnect is veryhigh and can open up a multitude of new smaller, low-cost and featurerich MEMS products. The following describes conventional methods forbonding and their problems. Organic or Adhesive Based Methods

Materials such as Benzocyclobutene (BCB), polyamide, photo resists,patternable RTV, and others have been spun on and used to form permanentbonds between wafers. These materials have disadvantages in that becausethey are organic, they tend to outgas and so are unsuitable for forminghermetic enclosures, and also they are susceptible to solvents, ormoistures which can lead to problems with long term reliability anddrift of a device's performance. Additionally, they are insulatingmaterials and so are incapable of forming a conductive path between twosubstrates.

One popular method of making wafer-wafer bonding is by use of fritglass. Frit glass is typically screen printed on the cover wafers andref lowed to form a patterned glass interface for subsequent wafer-waferbonding. Frit glass has a typical melting point near 500° C. and can beremelted post wafer-wafer aligned bond in a special temperature chamberwith a controlled environment. The primary use of glass frit is toprovide for the cover substrate and a hermetic sealed cavity for theMEMS. Frit glass technology has been utilized in the MEMS industry formany decades. Several major drawbacks are that frit glass does notprovide for electrical interconnection between the MEMS and cover, toachieve a hermetic seal interface, minimum of 400 micron seal ring widthis required which makes small MEMS devices, such as resonators andoptical devices, much larger than otherwise. Also, frit glass screenprinted is inherently a thick film process with tens of microns inthickness and several microns of nonuniformity.

Metal-Metal Bonding

CMOS compatible eutectic bonding has been demonstrated with indium-gold,solder-gold, gold-gold, etc. In order to bond a CMOS wafer, all of theseprior art systems require the addition of non-standard layers, such asplating of lead, indium, gold, etc., to be added to the CMOS wafer.Although these processes are capable of hermetic seals and electricalinterface, achieving fine features, small gaps and wafer uniformity isvery challenging and will result in yield losses.

There are many MEMS device applications that require anelectro-mechanical interface between the CMOS substrate and the MEMSsubstrates that are in micron gaps and require submicron uniformity.Most plating processes require under-layer barrier metalization withtens of microns thickness, and uniformity across the wafer is measuredin microns. Hence it is not possible to specify one or two micron gapcontrols between the MEMS and CMOS substrates using this bondingmethodology.

The ability to make high density and reliable electrical contactsbetween the MEMS and CMOS substrates can be very beneficial and providefor an all new generation of MEMS devices with much added functionality,smart electronics, smaller size, and lower cost. Finally, it isimportant to provide a lead free alloy based upon environmentalconsiderations.

Accordingly, what is needed is a system and method for providing waferbonding that overcomes the above-identified problems. The system andmethod should be easily implemented, cost effective and adaptable toexisting bonding processes. The present invention addresses such a need.

SUMMARY OF THE INVENTION

A method of bonding two substrates to create a robust electrical andmechanical contact by using aluminum and germanium eutectic alloys isdisclosed. An aluminum-germanium bond has the following uniquecombination of attributes: (1) it can form a hermetic seal; (2) it canbe used to create an electrically conductive path between twosubstrates; (3) it can be patterned so that this conduction path islocalized; (4) the bond can be made with the aluminum that is availableas a standard foundry CMOS process; (5) this process is compatible withcompletely fabricated CMOS wafers as post process; (6) this process canprovide for high density electrical interconnect; and (7) this processis highly controllable and provides for the smallest gap between twosubstrates. This has the significant advantage of allowing forwafer-level bonding or packaging without the addition of any additionalprocess layers to the CMOS wafer.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a flow chart of a method for fabrication of a wafer levelpackage in accordance with the present invention.

FIGS. 2A and 2B are cross sectional and top views of a first embodimentof an assembly in accordance with the present invention.

FIGS. 3A and 3B are cross sectional and top views of a second embodimentof an assembly in accordance with the present invention.

FIG. 4 illustrates an exemplary bonding profile to achieve a properAl/Ge bond.

DETAILED DESCRIPTION

The present invention relates generally to wafer bonding and moreparticularly to a method and system of bonding utilizing aluminum andgermanium in a wafer-level packaging of MEMS devices with electricalsubstrate interconnect environment. The following description ispresented to enable one of ordinary skill in the art to make and use theinvention and is provided in the context of a patent application and itsrequirements. Various modifications to the preferred embodiments and thegeneric principles and features described herein will be readilyapparent to those skilled in the art. Thus, the present invention is notintended to be limited to the embodiments shown, but is to be accordedthe widest scope consistent with the principles and features describedherein.

FIG. 1 is a flow chart of a method of fabrication of a wafer levelpackage in accordance with the present invention. The method comprisesproviding a MEMS structure including a substantially germanium toplayer, via step 12, and providing a CMOS structure including asubstantially aluminum top layer, via step 14. Finally, the methodcomprises bonding the top layer of the MEMS structure with the top layerof the CMOS structure, via step 16.

The following describes a preferred embodiment in accordance with thepresent invention. FIGS. 2A and 2B are cross sectional and top views ofa first embodiment of an assembly 100 in accordance with the presentinvention. Referring to the embodiment shown in FIG. 2A, a standardfoundry CMOS wafer 104 which includes aluminum is bonded to a MEMSsubstrate 102 which includes germanium to provide an aluminum/germanium(Al/Ge) bond 110. In this embodiment, a cavity 106 is within thesubstrate 104. The CMOS substrate wafer 104 can be any substrate withpatterned aluminum shown in FIG. 2B that is designed to interface withthe MEMS substrate 102 to make for a complete functioning product. Inaddition, a plurality of aluminum contacts 116 are on the top of theCMOS substrate 104 which are coupled to bond pads 105 by interconnect107. Vias 107 are provided in both the bond pads 105 and the aluminumcontacts 116 to allow for electrical connection thereto. As an example,the substrate 104 can comprise only a collection of metal layers andinterconnects for providing for an electrical interconnection to theMEMS layers. Furthermore, the MEMS substrate 102 includes a MEMS feature108 to complement the MEMS layers on the MEMS substrates 104, such ascorresponding cavity 106. A gap control standoff 111 is provided toprovide a precise separation of the MEMS substrate 102 from the CMOSsubstrate 104. The gap control standoff 111 provides a seal ring 112 forthe device.

FIGS. 3A and 3D are cross sectional and top views of a second embodimentof an assembly in accordance with the present invention. The assembly200 includes many of the same elements as assembly 100 of FIGS. 2A and2B and those elements have the same reference numerals. Additionally,the assembly 200 has via contacts 202 through the MEMS substrate 102′and the gap control standoff 111′ to provide electric feedthrough ofsignals.

Another important feature of the substrate 104 is the availability ofthe multilayer metallization standard in CMOS foundries withchemical-mechanical-polishing of the oxide to make for a very planarmetallized layer suitable for forming Al/Ge eutectic alloy with agermanium presence on the MEMS layer. The MEMS substrate 102 can be asilicon wafer or combination of silicon wafers assembled with all theMEMS features and functionalities including any type of preprocessedfeatures.

In the preferred embodiment, the MEMS substrate on which the germaniumhas been patterned is a silicon substrate doped with boron to aconductivity of 0.006-0.020 cm. This p+ doping forms an ohmic contactwith the aluminum-germanium eutectic mix following the bond.

To describe the bonding layers in more detail refer now to thefollowing.

Bonding Layers

In a preferred embodiment, the top metal layer of the foundry CMOS waferis a ratio mix of 97.5:2:0.5 Al:Si:Cu and is 700 nm thick and is on aplanarized oxide layer using CMP which is the standard processing stepfor most CMOS processes of 0.5 um or lower geometries. In a preferredembodiment, the bonding layers on the MEMS are 500 nm of germaniumdeposited in a standard vacuum sputter deposition system which isproperly patterned to match the corresponding aluminum patterned forbonding.

Below is an example of the preferred embodiment of the equipment andprocess in accordance with the present invention.

Required Equipment

The bonding is performed in a commercially available wafer bonder suchas that supplied by Electronic Visions Group, Inc., or Suss Microtec,Inc. The equipment should meet the following standards and have thefollowing capabilities: (1) temperature control of both the top and thebottom chuck to nominally 4500 C; (2) ambient pressure control tosub-tor; (3) ambient gas control (via a purge line); (4) plumbed with4-3-5 percent forming gas; and (5) the capability of applying a uniformforce across the wafer pair of a minimum of 3000N.

In the preferred embodiment, the wafers are pre cleaned and then alignedprior to bonding in a compatible alignment tool.

Pre-Bond Cleaning

In the preferred embodiment, both the CMOS wafer and the MEMS wafer arecleaned prior to bonding. Both wafers are assumed to be free of anyphotoresist or other extraneous materials from previous processingsteps. The wafers are cleaned by: (1) a 1.30 second dip in deionizedwater, (2) a 1.30 second dip in 50:1 HF; (3) a dump rinse; and (4) astandard spin-rinse-dry process.

Alignment

The bonding pair is aligned in an Electronic Visions 620 wafer-waferaligner. Separation flags are inserted to maintain separation of thebonded pair prior to bonding.

Bonding

The aligned pair is transferred to an Electronic Visions 501 bonder. Thepurge line of this machine has been plumbed with forming gas. Followingthe cool down period of the bonding recipe, the bonding is complete andrequires no further processing. An example of an exemplary bondingtemperature profile for achieving the proper Al/Ge bond is shown in FIG.4.

DESCRIPTION OF VARIOUS AND ALTERNATE EMBODIMENTS

Alternative embodiments include, for example, (1) the utilization ofdifferent materials on top of the germanium to protect it duringsubsequent MEMS processing; (2) the employment of different pre-bondcleaning methods; (3) the bond may be performed unaligned; (4) the bondmay be performed without patterning the aluminum and/or the germanium;(5) the CMOS wafer may be bonded without any additional processing otherthan the pre-bond clean; (6) the aluminum-germanium bond may beconfigured so as not to create a hermetic seal; (7) utilization of asubstrate other than a MEMS wafer (such as a simple cover wafer); (8)the MEMS substrate may comprise something other than a gyroscope (suchas a pressure sensor or accelerometer); (9) the aluminum of the standardCMOS wafer may comprise different formulations of standard aluminum (2%silicon, 2% silicon/1% copper, etc.)

In addition, (10) a specific temperature profile can be utilized; (11) aforming gas can be used to deoxidize the contact surfaces; (12) aluminumcan be utilized as the standard metallization used for IC fabrication;(13) aluminum substrate can be kept below a predetermined temperature toprevent complete leaching of aluminum and germanium alloy from the oxideon the substrate; (14) the bonding can be performed using controlledambient such as forming gas; (15) the bonding can be performed using lowpressure bonding force or high pressure bonding force as assistancefor/in order to assist in breaking of the aluminum oxide to initiate theinteraction; (16) two wafers can be prealigned prior to bonding process;(17) a special cleaning solution can be utilized to clean the oxide fromboth surfaces; (18) the bonding surfaces can be cleaned by sputteretching; (19) a thin layer of TiW can be utilized to protect the bondingsurface during MEMS processing; (20) including pre bond cleaning usingplasma and/or other insitu cleaning techniques, in addition to a moreconcentrated forming gas and higher force bond; (21) the germanium canbe deposited on a non-conductive layer (such as silicon dioxide) tocreate an insulating contact.

The germanium can be deposited on a semiconductor substrate that hasbeen doped such that the resulting contact between the aluminum of thesubstrate and the MEMS is rectifying. This substrate can be a siliconsubstrate with n-type doping to 0.020.05 Q-cm.

The germanium can be deposited on a semiconductor substrate that hasbeen doped such that the resulting contact between the aluminum of thesubstrate and the MEMS is ohmic.

A method and structure of bonding of germanium to aluminum between twosubstrates to create a robust electrical and mechanical contact isdisclosed. An aluminum-germanium bond has the following uniquecombination of attributes: (1) it can form a hermetic seal; (2) it canbe used to create an electrically conductive path between twosubstrates; (3) it can be patterned so that this conduction path islocalized; (4) the bond can be made with the aluminum that is availableas a standard foundry CMOS process. This has the significant advantageof allowing for wafer-level bonding or packaging without the addition ofany additional process layers to the CMOS wafer.

Although the present invention has been described in accordance with theembodiments shown, one of ordinary skill in the art will readilyrecognize that there could be variations to the embodiments and thosevariations would be within the spirit and scope of the presentinvention. Accordingly, many modifications may be made by one ofordinary skill in the art without departing from the spirit and scope ofthe appended claims.

What is claimed is:
 1. A MEMS device comprising: a first substrateincluding a microelectromechanical systems (MEMS) feature and apatterned germanium layer; and a second substrate, including a patternedaluminum layer, wherein the germanium of the patterned germanium layerof the first substrate is in direct contact with and matched to thealuminum of the patterned aluminum layer of the second substrate to forma contact area, wherein in the contact area the patterned aluminum layeris properly patterned to match patterned germanium layer.
 2. The MEMSdevice of claim 1, wherein the second substrate includes electricalcircuits.
 3. The MEMS device of claim 1, wherein the first and secondsubstrates are electrically connected.
 4. The MEMS device of claim 1,wherein the patterned aluminum layer and the patterned germanium layerform a seal ring.
 5. The MEMS device of claim 1, wherein the directcontact between germanium and the aluminum comprises a eutectic bond.